Electrical circuitry often should be protected from electromagnetic interference (EMI) and radio frequency interference (RFI) emanating from or impinging on the electrical circuitry. Although EMI and RFI often are referred to interchangeably, EMI has been used to connote energy occurring anywhere in the electromagnetic spectrum whereas RFI tends to connote interference in the radio communication band. EMI energy can be generated outside as well as inside the electrical circuitry. External EMI energy can interfere with the operation of the electrical circuitry or electronic equipment coupled thereto, while internal EMI energy can create "cross talk" and "noise" which can cause errors in the signals, such as data, transmitted through the electrical circuitry.
Electrical connectors are particularly prone to problems caused by EMI energy because of the density of contacts within the connectors and the openings in the connectors for electrical terminals and cables. While various electrical connectors have been designed with shielding that is effective against EMI/RFI energy, it often is desirable to shield the cables extending to the connectors as well as shielding the connectors themselves.
One type of cable used to reduce the effects from interference is referred to as a "twisted pair" cable. This type of cable includes two adjacent conductors or differential pairs twisted with respect to each other so that the lateral position of each conductor is reversed at each twist. In a given differential pair, electric currents flow in opposite directions in each of the conductors so that the benefits of the differential pair configuration are twofold.
First, the relative position of the conductors with respect to each other is constantly being reversed. As a result, any exterior magnetic or electric field in the vicinity of the twisted pair of conductors has a generally uniform effect upon that differential pair of conductors. In view of the fact that the current is flowing in opposite directions in the signal conductors of the differential pair and the impact of an induced or coupled noise component is generally uniform on both of those signal conductors and any harmful effects from exterior electromagnetic fields is reduced, if not eliminated, by stripping this common mode noise from the differential signal thereby lessening the chances that errors will be introduced into the data being transmitted on those signal conductors.
Second, an electromagnetic field is generated when current runs through a conductor. The orientation of that field is dependent upon the direction of the current flow in the conductor. With the constant changing of the juxtaposition of the conductors that form the differential signal pair, the field orientation for a given region is constantly being reversed and can be considered self-canceling. This canceling effect can substantially suppress the radiated emissions from a given differential pair.
FPC is another medium used for high speed data transmission between computers and the peripherals connected thereto. FPC is typically formed using a process in which a conductor such as copper is deposited uniformly over a flexible insulator substrate. A mask in a desired pattern is then applied to the conductor and the conductor is chemically removed everywhere except at the location of the mask. When the mask is removed, only the conductor remains in the desired pattern on the substrate. An insulator, such as tape or a flexible film, is applied over the conductor and to the flexible substrate in order that the conductor is sandwiched between two insulators.
While conductors on the same side of the insulator substrate of a FPC cannot be crossed, a "pseudo-twisted" arrangement can be achieved in a FPC by placing conductors of a given pair on opposite sides of the insulator substrate. The paths of these conductors are slightly and oppositely offset with respect to a common nominal path and this offset is periodically reversed at predetermined locations. An example of a "pseudo-twisted" FPC arrangement is shown in U.S. Pat. No. 3,761,842, dated Sep. 25, 1973. Another example of a "pseudo-twisted" FPC arrangement is shown in U.S. Pat. No. 5,939,952, dated Aug. 17, 1999 and assigned to the assignee of the present invention.
Pseudo-twisted FPC of the prior art typically has not included a grounding system when the cable is of a two layered construction having only two conductive layers, or when the cable has each of the pair of conductors on an opposite side of an insulative carrier or substrate. Such a prior art flexible printed circuit or FPC is illustrated in FIGS. 1 and 2 of the drawings and is generally designated by the reference numeral 1. The FPC 1 includes a flexible dielectric substrate 2 on opposite sides of which is disposed a plurality of pairs of pseudo-twisted conductors 3a and 3b. Conductor 3a is disposed on one side or surface of the flexible dielectric substrate 2 and the other conductor 3b is disposed on the opposite other side or surface of the substrate 2. An insulative film or coating can be disposed over the conductors 3a and 3b or can be disposed over the entire opposite surfaces of the flexible dielectric substrate 2 such that the conductors 3a and 3b will be covered by the film. In this way, each of the conductors 3a and 3b is sandwiched between the flexible substrate 2 and the protective film covering the surface on which the conductors 3a and 3b are disposed.
Each conductor 3a and 3b runs lengthwise of the FPC 1 in an oscillating pattern formed by alternate straight sections 4 and oblique sections 5. As a result, these conductors 3a and 3b extend in a periodic pattern symmetrically with respect to each other but on opposite sides or surfaces of the flexible substrate 2. The straight sections 4 of each conductor 3a and 3b are generally parallel to each other, but alternate along two parallel but spaced apart lines 4a and 4b (FIG. 1). The oblique sections 5 of each of the conductors 3a and 3b connects a pair of adjacent straight sections 4 and thus extends generally between straight-to-oblique transfer points or intersections 7 located at the lines 4a and 4b. In view of the fact that the conductors 3a and 3b are arranged symmetrically but opposite to each other, the straight sections 4 of the conductors 3a and 3b extend longitudinally generally parallel to each other but spaced apart laterally with respect to each other (left. to right as the sections 4 are viewed in FIG. 1). Consequently, the straight section 4 of the conductor 3a on one side of the substrate 2 is in alignment with the line 4a when the straight section 4 of the other conductor 3b on the opposite side of the substrate 2 is in alignment with the line 4b, and vice versa. As a result, the oblique sections 5 of the conductors 3a and 3b cross each other on opposite sides of the substrate 2 at crossover points 8.
Each of the conductors 3a and 3b separately terminates in one of a plurality of pads 9 (FIGS. 1-2) at edges 2a and 2b of the flexible substrate 2. The pads 9 of some of the conductors (for example, the conductor 3b) may include an associated through-hole or via 10. These pads 9 are arranged at equal intervals along the edges 2a and 2b of the FPC 1 for engagement with equally spaced contacts of an associated electrical connector. Therefore, either opposite edge of the FPC 1 having the pseudo-twisted conductors 3a and 3b can be inserted into such an electrical connector to establish a required electrical connection.
In the prior art FPC illustrated in FIGS. 1-2, the straight sections 4 of conductors 3a and 3b have uniform or equal widths along the lengths thereof. In an alternate embodiment, the length of the straight sections 4 could vary as is shown in U.S. Pat. No. 6,057,512, dated May 2, 2000 and assigned to the assignee of the present invention. Each oblique section 5 of each of the conductors 3a and 3b of the prior art FPC 1 disclosed in FIGS. 1-2 decreases in width uniformly in a direction from the straight-to-oblique transfer point or intersection 7 of one of the conductors 3a and 3b to the respective crossover point 8 of those conductors 3a and 3b where the oblique section 5 reaches its minimum width. Assuming that 1) the thickness of conductors 3a and 3b (i.e., in a direction perpendicular to the plane of the FPC 1) is uniform along the full length of the conductors 3a and 3b; 2) the distance in the same perpendicular direction between the conductors 3a and 3b remains constant; and 3) the dielectric constant of the material between the conductors 3a and 3b (i.e., the substrate 2) remains constant, the impedance of the conductors 3a and 3b increases due to the decrease in the width of each oblique section 5.
On the other hand, the impedance of those sections 5 also decreases because the conductor-to-conductor centerline distance between oblique sections 5 of conductors 3a and 3b in each pseudo-twisted pair decreases as the oblique sections 5 extend closer to each other at each crossover point 8. However, each oblique section 5 is uniformly reduced in width as it extends toward the cross over point 8 such that the impedance is caused to uniformly increase. In other words, the conductor-to-conductor centerline distance decreases between the oblique sections 5 of each pair of conductors 3a and 3b as they approach each other at each of the crossover points 8 such that the impedance of the conductors 3a and 3b is decreased. However, the width or lateral dimension of the oblique sections 5 also decreases as they approach the crossover points 8 such that the impedance of the conductors 3a and 3b uniformly increases. In essence, the impedance of the conductors 3a and 3b simultaneously decreases due to the reduction in the relative distance between the centerlines of the conductors 3a and 3b in the oblique sections 5 and increases due to the gradual tapering or narrowing of the conductors 3a and 3b in those oblique sections 5. This simultaneous decrease and increase in the impedances of the conductors 3a and 3b results in the impedance remaining substantially unchanged in the oblique sections 5 as they extend from the straight-to-oblique transfer points 7 to the crossover points 8. The impedance in the oblique sections 5 is preferably set to be equal to the impedance in the parallel extending sections 4 of the pseudo-twisted conductors 3a and 3b so that the pseudo-twisted FPC 1 has a relatively constant impedance along its entire length.
When a FPC is operating under ideal conditions, a return path or ground (sometimes referred to as a "virtual ground") is not necessarily needed. However, the signal in one conductor can lead the signal in the other conductor so that the signals are skewed or unbalanced. This tends to occur when the signals are being transmitted over an extended period of time, or over an extended distance or at a high speed data transmission rate. Problems associated with such skewed or unbalanced signals would be reduced if a ground reference is provided with respect to the conductors.
Heretofore, grounding grids have been used with FPC where a ground reference is needed. These grids constitute a ground plane of a conductive wire mesh formed by a plurality of ground conductors which crisscross each other to define open spaces therebetween. However, such grounding grids add an additional layer or layers to the two layered FPC. This increases the complexity of the FPC and thus makes it more expensive to manufacture. An example of a FPC with a ground grid arrangement is shown in co-pending application Ser. No. 08/932,545 filed Sept. 17, 1997 and assigned to the assignee of the present invention.
Consequently, it would be advantageous to provide a ground system in a FPC without the necessity of adding an additional layer to the FPC. Such a ground system would be particularly useful in a FPC that has conductors on opposite sides of an insulating substrate to form a pseudo-twisted flat FPC.